1. Field of the Invention
This invention relates to optical information play-back apparatus, and more particularly to apparatus for playing back information recorded on an optical video disk.
2. Description of the Prior Art
In general, information play-back apparatus, for example, video disk play-back apparatus requires in order to optically and accurately play back information recorded on a video disk, auto-focusing means for focusing a laser beam so that it may always converge on the video disk, and tracking means for controlling the position of the converging point of the laser beam so that the laser beam may always track a predetermined information track on the disk.
Hereunder, problems in prior arts will be described on the auto-focusing and then on the tracking.
As a typical auto-focusing method having heretofore been employed, there is one in which using a light beam for detecting a focal position besides a light beam for detecting information, the variation of a disk on the optical axis is detected, and the converging point of the light beam for playing back information is brought onto the disk automatically with the detected signal. Since, however, the prior-art auto-focusing method requires two optical systems, it involves a complicated structure.
When, in the above method requiring the two light beams, a single light source therefor is used, a laser source of high power is necessary. When two light sources are employed, the apparatus becomes expensive.
In order to eliminate the drawbacks, apparatus as shown in FIG. 1 has been proposed. This apparatus executes the auto-focusing by the use of an astigmatic optical system. That is, using an optical element having the unidirectional lens action (for example, a cylindrical lens), defocusing is detected in the form of a change in the optical density distribution of a reflected laser beam.
Referring to FIG. 1, a laser beam 2 emergent from a laser source (for example, He-Ne laser or semiconductor laser) 1 passes through a beam splitter 3 and a converging lens 4 and is converged on an information track 6 of a video disk 5. A reflected laser beam containing information of the information track 6 passes through the converging lens 4, is reflected by the beam splitter 3 and is converged only in one direction by a cylindrical lens 7. Thus, it is converted into a non-point convergent beam.
Accordingly, the contour of the reflected laser beam becomes elongate in an X-direction at a point 8 (a point on which the reflected laser beam is focused by the converging lens 4 and the cylindrical lens 7), and it becomes elongate in a y-direction at a point 9 (the focus of the converging lens 4). At an intermediate point 10 (a point which lies substantially at the middle between the points 8 and 9), the reflected laser beam exhibits a circular contour which extends substantially equally in the x- and y-directions. If the video disk 5 shifts in a direction in which it comes close to the converging lens 4, the contour of the reflected laser beam at the point 10 becomes elongate in the x-direction. Conversely, if the video disk 5 shifts in a direction in which it goes away from the converging lens 4, the contour of the reflected laser beam at the point 10 becomes elongate in the y-direction.
FIG. 2 is a schematic block diagram in the case where a photodetector 11 having four photocells 11a-11d is arranged at the position of the point 10 indicated in FIG. 1. Signals of two ones 11a and 11c of the four photocells of the photodetector 11 are added by an adder 12, while signals of the other two photocells 11b and 11d are added by an adder 13. Outputs from the two adders 12 and 13 are applied to a differential amplifier 14. Further, an output from the differential amplifier 14 is applied to a driver 15 for the converging lens 4. The driver 15 serves to move the converging lens 4 in the direction in which it comes close to or goes away from the video disk 5, in response to the output of the differential amplifier 14. More specifically, in case where the illumination beam is exactly focused on the video disk 5, the contour of the reflected laser beam on the photodetector 11 becomes a circular pattern which extends equally in the x- and y-directions as indicated by a solid line (a), and the light intensity is not maldistributed, so that the output of the differential amplifier 14 becomes zero. Therefore, the converging lens 4 is not moved. If the video disk 5 shifts in the direction in which it comes close to the converging lens 4, the contour of the reflected laser beam becomes elongate in the x-direction as indicated by a dotted line (b). In consequence, the output of the differential amplifier 14 becomes negative, and the converging lens 4 is moved in the direction of going away from the video disk 5. Conversely, if the video disk 5 goes away from the converging lens 4, the contour becomes elongate in the y-direction as indicated by a dotted line (c). In consequence, the output of the differential amplifier 14 becomes positive, and the converging lens 4 is moved in the direction of coming close to the video disk 5.
The auto-focusing of the proposed system, however, has the disadvantage that an unbalance in the light intensity distribution of the reflected laser beam arises under the influence of a pit in the information track and that the unbalance degrades the precision of the auto-focusing. That is, even in the case of the circular contour which extends equally in the x- and y-directions as indicated by the solid line (a), the output of the differential amplifier 14 does not become zero. Accordingly, notwithstanding that the illumination laser beam is in focus, the converging lens 4 is moved relative to the disk 5. The unbalance of the light intensity distribution will now be explained.
Information is written in the video disk 5 in the form of a pit which is 1 .mu.m wide, 2-3 .mu.m long and 0.15 .mu.m deep. Regarding the depth, a value of 1/4 of the wavelength of the play-back laser light is the optimum. It is ideal to make the depth of the pit exactly the quarter wavelength and to make the edge of the pit exactly 90.degree. with respect to the disk surface (to make the edge of the pit sharp). In the manufacture of the disk, however, it is difficult to form the pits at such high precision. When a spot of the laser beam illuminates the pit under the state under which the depth of the pit deviates from 1/4 of the wavelength of the play-back laser light and under which the spot is eccentric to the pit, the light intensity distribution on the photocells 11a-11d is unbalanced in the x-direction as in a spot 18 shown by way of example in FIG. 3.
When, even if the depth of the pit is the 1/4 wavelength, the laser beam does not impinge on the center of the pit, diffracted light ascribable to obtuseness of the edge (stepped part) of the pit gives rise to an unbalance in the light intensity distribution in the x-direction likewise to the case of the spot 18 shown by way of example in FIG. 3.
That is, a spot 17 of the reflected laser beam on the photodetector 11 having the photocells 11a-11d contains the diffracted light spot 18 which is maldistributed in the x-direction and which is attributed to the pit. Accordingly, the light distribution on the photodetector 11 is intenser in the x-direction than in the y-direction.
Although such phenomenon can occur in both the x- and y- directions, the unbalance in the light intensity distribution in the y-direction is not a serious problem. More specifically, in the video disk, information are successively read out in the direction of the track (in case of a circular track, in the tangential direction thereof). Therefore, supposing the direction of the track to be the y-direction, even when the diffracted beam becomes unbalanced in the y-direction at a certain time, the unbalance in the course in which the beam spot moves towards the central part of the pit and the unbalance in the course in which the beam spot departs therefrom appear in senses opposite to each other. Accordingly, the unbalances in the y-direction are averaged, and they hardly affect the automatic adjustment of focusing. In contrast, the unbalances in the x-direction appear in the same direction both when the beam spot comes close to the central part of the pit and when the beam spot goes away therefrom, so that they have an bad effect on the automatic adjustment of focusing.
That is, notwithstanding that the lens is in focus, the unbalances in the x-direction generate the output of the differential amplifier 14 and move the lens in a fixed sense.
On the other hand, as to the tracking, there have heretofore been several methods. For example, there has been apparatus in which two laser beams exclusively for the tracking are jointly used, or apparatus in which a laser beam is minutely vibrated on a track. Both the apparatuses, however, are disadvantageous in that a laser beam is required anew or that an element for the minute vibrations is required, so that the apparatus itself becomes complicated and large-scaled, which in turn makes the apparatus expensive.